Feature Flag Default Value Strategies

Default Value Strategies

The default value of a feature flag determines what happens when the flag system is unavailable, when a user doesn't match any targeting rules, or when the flag is evaluated for the first time. Choosing the right default is critical for system reliability.

The Golden Rule

The default value should represent the safest, most conservative behavior.

For new features: default to OFF (don't show the feature). For kill switches: default to ON (keep the system running).

Configuration Examples

{
  "new-feature": {
    "type": "boolean",
    "defaultValue": false,
    "description": "New feature -- safe to hide if flag service is down"
  },
  "payment-enabled": {
    "type": "boolean",
    "defaultValue": true,
    "description": "Kill switch -- must work even if flag service is down"
  },
  "api-timeout-ms": {
    "type": "number",
    "defaultValue": 5000,
    "description": "Conservative timeout -- higher default is safer"
  },
  "theme": {
    "type": "string",
    "defaultValue": "system",
    "description": "Follows system preference if flag service is unavailable"
  }
}

Default Value Decision Matrix

Flag Purpose	Type	Recommended Default	Reasoning
New feature	boolean	false	Don't expose incomplete features
Kill switch	boolean	true	Service should work without flags
Rate limit	number	Conservative value	Higher limits are safer than lower
Timeout	number	Higher value	Better slow than failing
UI variant	string	Current/original	Don't change UX unexpectedly
Config object	json	Minimal safe config	Include only required fields

SDK-Level Defaults

Most SDKs let you specify a fallback when evaluating a flag:

// Application-level default as final fallback
const showFeature = client.boolVariation(
  "new-feature",
  user,
  false // SDK-level fallback if flag not found
);

Fallback Chain

The evaluation order for defaults:

1. Targeting rules match → Rule's variation value
2. No rules match, flag ON → Flag's fallthrough value
3. Flag is OFF → Flag's off variation
4. Flag not found → SDK-level default parameter
5. SDK not initialized → Application hardcoded default

Common Mistakes

• Setting a new feature's default to true (exposed before ready)
• Using 0 as default for rate limits (blocks all requests)
• Empty string defaults for required string configs
• Not testing what happens when the flag service is completely unavailable